Image rights holders may post images in professional image banks, where they can be viewed and licensed by others. Professional image banks will often utilize technologies to mark digital images so that their proper use can be monitored and charged for, and their unauthorized use identified. For example, image banks may watermark an image by embedding special codes directly into the image. By searching for images with watermarks, the professional image bank can identify uses of images and assess whether such uses were authorized or unauthorized by the rights holder.
While image protection schemes are typically very effective when images are copied and used directly, many protection schemes can be defeated if changes are made to the image. For example, it can be difficult to detect images that have been cropped, resized or partially modified. In addition, it may be very difficult for typical protection schemes to identify unauthorized digital images that are scanned copies of printed originals. For example, images provided by newspapers or magazines may be scanned and then posted on the Internet in a manner that makes them very difficult to detect.
Some existing image comparison systems may match an image for different versions of the same image. However these systems typically have limited effectiveness identifying redigitized versions of images. These image comparison systems are particularly ineffective if the image has been cropped, flipped, rotated, the color balance has been changed and/or if other distortions have been applied.